if it is showing padlock when trying to boot to an external drive it means there is a firmware password - if you know the password enter it and boot to dvd if you don't know the password you need to change the RAM configuration and reset the PRAM to remove the firmware password altogether. you can find info on this on apple support pages Change RAM config = basically take a stick of ram out of the machine Reset PRAM = with ONE RAM stick removed, switch on the machine HOLDING DOWN BUTTONS CMD + ALT + P + R wait for the machine to reboot a few times. put ram stick back in and boot to external drive normally.
My machine had red flashing light in the bottom right - E47 error appeared on screen Most online suggestions were to unplug all power cables and harddrive and reconnect - did this a bunch of times and issue persisted. Bought the X-Clamp kit off eBay for a few quid and replaced them on my machine - after a few re-boots it now works perfectly fine. not sure it's a permanent fix tho. IF you're yet to take the machine apart you may qualify for the E47 warranty extension from Microsoft, double check that first. If not try XClamp fix, worst comes to worst you're still gonna have a broken xbox :P